This invention relates generally to the plating of metal on a plastic surface, and more particularly to methods for chemically treating a thermoplastic substrate upon which a printed circuit is formed by various plating and patterning techniques.
Printed circuit boards are now the predominant vehicle for mounting and interconnecting electronic components to form a desired electronic circuit. The printed circuit board typically consists of a sheet of a dielectric substrate constructed of various filled or unfilled synthetic materials such as phenolics and glass-impregnated epoxies. The substrate is usually provided with a pattern of thin metal foil which functions as a conductive path or "trace" on one or both sides. The traces collectively define all of the electrical connections between components on the board, and are routed between appropriate locations.
Traditionally, printed circuit boards have been produced by laminating a metal such as copper to one or both sides of a reinforced plastic board. "Through-holes" are then drilled through the board, followed by catalysis of the surface surrounding the through-hole in preparation for electroless plating with a metal. After the electroless deposition, the metal surface is patterned, imaged, and developed. The metal thickness is then built up on the board surface and in the holes by electroplating, followed by etching and removal of the photoresist to produce the desired circuit pattern. The etching mask is often a tin/lead mixture. In this type of circuit board production, the precious metal catalyst never contacts the background regions of the substrate, i.e., the regions which are free of metal in the final product.
In recent years, the use of thermoplastic substrates such as polysulfones and polyimides has considerably advanced printed circuit board technology. These materials are particularly suitable because of their strength, heat resistance, dimensional stability, and easy moldability. In this process, the cumbersome metal lamination step is eliminated because the plastic substrates can be metallized directly by electroless plating. Furthermore, the need for drilling through-holes is also eliminated because such holes are already formed during the plastic molding process. The disadvantages attendant with drilling, such as the presence of melted plastic drilling debris ("smear"), and the general degradation of the plastic beyond the drilled edge, are also advantageously eliminated.
While the advent of directly-metallized plastic circuit board substrates has greatly enhanced the art, several new problems have arisen. For example, in preparation for electroless plating, a precious metal catalyst is employed to activate the plastic surface. After plating and patterning of the circuit metal are accomplished, portions of the catalyst remain tightly adherent to the substrate. Since the catalyst metal is conductive, it can function as an electrical pathway between adjacent traces in the circuit pattern, thereby resulting in current leakage during operation of the circuit. Furthermore, additional layers of metal subsequently applied to the substrate may preferentially adhere to areas which include the residual catalyst, thereby inviting current leakage or short circuits. In summary, the presence of residual plating catalysts can greatly impair the integrity of the electrical circuit.
Attempts to remove the residual precious metal catalyst unfortunately invite other complications. For example, the etching solutions used to remove selected portions of the plated metal are not aggressive enough to remove the catalyst, which is typically a fairly nonreactive metal such as palladium. More aggressive solutions could remove the residual catalyst, but only by sacrificing portions of the desired metal pattern. Furthermore, strong cleaning solutions, such as those which are highly acidic or highly basic, can chemically attack both the etching mask and the underlying plastic substrate, thereby damaging the circuit board.
It is therefore the primary objective of the present invention to provide a method for removing residual precious metal catalyst from the surface of a plastic circuit board without degrading the printed circuit.
An additional objective of this invention is to provide a method for precious metal catalyst removal which does not result in damage to plastic material in contact with the precious metal.
A further objective of the present invention is to provide a method for forming a high quality circuit pattern of metal directly applied to a plastic substrate.